Roll src/third_party/skia 18f5b1a6dd77..a1991f5e3b58 (6 commits)

https://skia.googlesource.com/skia.git/+log/18f5b1a6dd77..a1991f5e3b58

git log 18f5b1a6dd77..a1991f5e3b58 --date=short --no-merges --format='%ad %ae %s'
2019-10-28 mtklein@google.com experimental support for go/fibers
2019-10-28 robertphillips@google.com Add helper to determine the index buffer that will be used
2019-10-28 jcgregorio@google.com Revert "Add variety of bulk API benchmarks"
2019-10-28 kjlubick@google.com [canvaskit] Use skia's version of png and zlib
2019-10-28 michaelludwig@google.com Add variety of bulk API benchmarks
2019-10-28 kjlubick@google.com [canvaskit] Use newer freetype

Created with:
  gclient setdep -r src/third_party/skia@a1991f5e3b58

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/skia-autoroll
Please CC jcgregorio@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

CQ_INCLUDE_TRYBOTS=luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux-blink-rel;luci.chromium.try:linux-chromeos-compile-dbg;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
TBR=jcgregorio@google.com

Bug: None
Change-Id: I04dd91f1d67ee96330ac2abc8cbc570f71d88498
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884431Reviewed-by: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Commit-Queue: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#710003}

DEPS

@@ -171,7 +171,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling Skia
   # and whatever else without interference from each other.
-  'skia_revision': '18f5b1a6dd77adcb52ff70a4ae8139167de92b48',
+  'skia_revision': 'a1991f5e3b58479372d7091d97611c5004585d68',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.